The Astronomy Mega Thread (More than 50 pages of info!)

[saundby]

Just a couple of notes on Pluto's "demotion."

There's more at play than American nationalism in the political side of the debate. French nationalism has also appeared. There's also the fact that some are swayed by varying interpretations of the effects different classification systems will have on obtaining funding for projects that involve Pluto and similar bodies. Some feel that having Pluto classified as something other than a planet will make it easier to get project funding to study it, since they won't be pitted against projects for planets like Jupiter and Saturn. Others think it'll be easier to get funding if it keeps planet status.

Aside from the political battles, the IAU process that resulted in the recent decision was flawed at just about every stage of the game. Sure, the claim can be made that the "letter of the law" was followed, but that only shows the problems with the official process. Not everyone who should have been was involved before the meeting, one of the major outcomes being the problems the dynamicists had with the models that had been prepared before the meeting. Another problem was the last-minute whip-up of a model and last-minute vote by a group that ended up with the same sort of problems with representation from appropriate parties. It would have been better to have no decision than a flawed one (IMO), with efforts for forming a better working group to prepare proposals for the next meeting. As it is, the decision stands to be revisited at the next IAU meeting anyway. This doesn't breed respect for the process or the participants among the public or among the funding bodies.

It's a darn shame it turned out this way. I hope things turn out better at the next IAU meeting (and there's better preparation for it.)

[VoijaRisa]

Spiral Structure

I’m not entirely sure why, but recently, in various forums I hang out on, I’ve been seeing a particular creationist argument pop up that’s fairly annoying. The argument is that the universe can’t be as old as astronomers say it is because, if it were, the spiral arms in galaxies would “wind up”.

This argument is pretty convincing at first, because it plays on the expectation that galaxies are fairly straightforward systems without dirty little tricks up their sleeves. If this were the case, galaxies would behave like the solar system wherein the things towards the center go around quickly and objects towards the edges slowly work their way around. Thus, if you started everything on a straight line, the inner part would have orbited several times while the outer edges would barely have budged thanks to the fact that those inner bits not only move faster, but have a shorter distance to travel in their smaller orbits.

Expectation wise, galaxies shouldn’t have graceful arms that tend to only go about half a full turn. Unless, of course, those expectations are wrong. And I’m sure by now most readers are anticipating what should be the obvious answer: They are.

The first thing that’s wrong is that galaxies don’t behave like our solar system. This was first realized in 1959 in the galaxy M33 by Louise Volders. In the 1970’s, this non-Keplarian motion was discovered in more and more galaxies by Vera Rubin, which is one of the foundations for the need for Dark Matter. Instead of velocities trailing off in an exponential decay like we see in the solar system, most galaxies tend to have fairly flat velocity curves as shown in Figure 1. A flat rotation curve means that galaxies don’t wind up and if they do, it would be very slowly.

Figure 1

Of course, this raises an even more interesting question. Even though we’ve established that these arms could be very old, it still doesn’t say anything about why you have regions in the galaxy where they exist in the first place. What’s so special about spiral structure!?

The answer to this question is profound because it gets to the very nature of what spiral arms are and, as with before, it’s something that completely defies what you’re probably thinking if you’re not already familiar with this topic: Spiral structure is the result of spiral density waves...

Whatever the hell that means.

Unfortunately, there’s no real good way I’ve yet come across to explain this. Silly analogies with traffic jams tell you what is going on, but says nothing of why it happens. And that “why” is fairly important, lest we be accused of making up just-so stories.

I’m going to skip the math since it requires a course in mechanics and quite a bit of calculus to go through and I’ll just get to the heart of what it reveals.

Let’s start off with an idealized galaxy in which we have everything as simple as we can get it. Let’s pretend the galaxy is a perfectly uniform distribution of gas and dust in perfectly circular orbits around the center. Yeah right. No galaxy really starts off that way. Galaxies are in clusters and being acted on by one another. Many galaxies have satellites that perturb them and they were formed from smaller lumps that have gotten cannibalized. They’re dynamic places, so if we introduce some perturbations to the system, we can ask what happens then.

No need to really do this for the whole galaxy at this point. We can just look at the case of what would happen to any single star. It ends up that if you knocked a star out of its ideal orbit, the combined forces of the rest of the galaxy would work to try to restore its ideal position. But as with most restorative forces, it will tend to overshoot. Thus, you get harmonic motion. Not only do you get this towards and away from the center of the galaxy though, you also get it in the direction of motion. The combination of these two motions means that the star would make little orbits about its idealized location. The Sun does this too. Its motion in relation to the idealized point it should be (known as the Local Standard of Rest or LSR) is known as “Peculiar Motion”.

So let’s try to picture what that looks like. In Figure 2, I’ve drawn the idealized LSR for our star in green. On top of that, I’ve sketched the path it would travel around an idealized point in red at four different points in the full orbit. The extent of this is grossly exaggerated of course, but it will illustrate the point.

Figure 2

Now let’s have the star make an integer number of rotations around its small circle every time it goes around the full big circle. If we take the star to be at its furthest from the center of the galaxy at point a, then it would be at its closest at b, furthest again at c, and closest again at d. Now putting the overall true motion of the star (in blue) in with the smaller wobbles, the true motion becomes apparent: Stars have elliptical orbits. This is shown in Figure 3

Figure 3

What’s more, the way these elliptical orbits line up isn’t the same at all distances from the center of the galaxy. Rather, the orientation of the major axis of the ellipse will rotate as you move out. That means that if we drew is successive ovals, each one would be slightly skewed resulting in a pattern like the one I’ve shown here.

Figure 4

The result of these skewed orbits is that at some points around the galaxy, stars tend to all bunch up more than at other places. And what happens when you get a bunch of massive things bunching up? It creates a gravitational potential. The result is that stars will speed up as they fall in making a lack of stars on the trailing edge but then be slowed down again as they exit, creating a pile up on the front edge. If spiral arms really were some magical place that could get wound up, we shouldn’t expect to see this. But we do, which suggests that the Creationist version of spiral arms doesn’t fit reality.

And this is what a spiral arm is. It’s the crowding together of a bunch of stuff as its slowed down exiting the potential well caused by the spiral wave. And it turns out these things are very stable. So the Creationist claim that they must be very young because galaxies should “wind up” is just wrong. It’s using a horribly outdated idea of what spiral structure is. Not that this is surprising.

But how robust is this explanation? Turns out it does very well to explain not just the very nice two armed spirals like M51, but it also can explain the not so neat spiral galaxies like M101. It just takes a different number of orbits around the small path per full orbit. And even the flocculent spirals with patchy arms can be explained through this, using non-integer numbers of rotations or linear combinations of many states.

I hope that clears up what’s really going on with spiral structure. If you’re really interested in seeing the math behind this, it should be in most junior/senior level astronomy texts.

[VoijaRisa]

Hidden Creation

One of the excuses I've seen creationists use as to why astronomy can't possibly be right (and thus magic man done it) is that we've "never seen a star form". The response I gave noted that there's 2 primary reasons for this.

1) Unlike star deaths, star births aren't expected to be sudden and dramatic. Even for the most massive stars which collapse the fastest, this takes tens to hundreds of thousands of years. For the less massive stars, this can take even longer! It's not a sudden flick of a light switch were it's not glowing one moment and there the next. Rather, it's a slow process of heating.

2) Stars form behind curtains of dust and gas. Dust and gas block light. They're especially good at blocking light in the regions we really like to look, namely the visible region of the spectrum. However, we can somewhat peer into these nebulae by looking at longer wavelengths which are better able to get through.

Recently, this was spectacularly confirmed by the Spitzer telescope which peered into a dusty patch of sky around Rho Ophiuchi. If an image is taken in the visible part of the spectrum, it looks like the first image here.



It's definitely pretty. The stars around the area have wonderfully contrasting colors that make it a popular target in small scopes. But one of the defining characteristics of this area that's overlooked is the dark band near the center of the image. It's dark, boring, and there's not a whole lot going on there. At least not at first glance.



But what's really going on is the process of star formation! When Spitzer peels back the veil of dust and gas, dozens of high mass, young stars are found in the exact region that looks devoid! Some of the stars even have the large circumstellar disks still present.

A careful inspection of various regions in this and the other image released by Spitzer show many other features associated with newly forming stars: jets of material slamming into the cloud in which they're forming, causing bow shocks known as Herbig-Haro objects.

So the creationist canard that we "never see stars form" is a bit of a silly statement. It doesn't happen quickly, but through techniques and telescopes like Spitzer used, we can see frozen vestiges of star formation.

Note: The images above are the same scale and orientation.

[carrot138]

VoijaRisa

Four Misconceptions About the Big Bang
a) Cosmological Redshift: As I explained in my earlier post, we can use spectroscopy to determine the rate at which galaxies are moving away from us. Additionally, since it takes light time to travel, the further away we look, the further back in time we are looking.

What we find, is that all galaxies in the universe are moving away from us. The further they are, the faster they’re moving away. So if we play the whole thing in reverse, all the galaxies will come back together at a single point in time. This point in time is what we call the Big Bang.

Wonderful thread! I just had one question about this part. Are the galaxies in our local cluster also moving away from us?

[VoijaRisa]

carrot138

Wonderful thread! I just had one question about this part. Are the galaxies in our local cluster also moving away from us?

Yes and no. Some are due to their inherent motions, but not due to the expansion of the universe which is what I think you're getting at.

In bound systems like super clusters, clusters, galaxies, and solar systems, gravitational attraction overcomes the universal expansion.

To use an analogy, if you've ever tried to spread pizza dough on a greased surface it tends to try to shrink back up. The tension in the dough overcomes the friction (which was is a result of you having spread it out).

The universe may do the spreading, but other forces can fix it on small scales.

[Katherine1]

Question: Other than through discrepancies between observation and gravitational calculations, what other evidence exists for dark matter?

[VoijaRisa]

Katherine1

Question: Other than through discrepancies between observation and gravitational calculations, what other evidence exists for dark matter?

Gravitational lensing as well as the Friedmann equation.